1. Field of the Invention
The present invention relates to a display signal conversion apparatus particularly to a signal conversion from an RGB input signal to an RGBW display signal.
2. Description of the Related Art
As described in “New-Edition Color Science Handbook, 2nd edition,” edited by the Color Science Association of Japan, University of Tokyo Press, originally printed in 1998, a color image may be constituted by RGB (red, green, and blue) three color signals. Further, a smallest unit of a color image is called a pixel. A screen that is a set of pixels is called image data, a video signal, a color signal, or the like depending on fields. In the following description, those terms may be used in a mixed manner.
Display devices based on various principles have been developed for the purpose of displaying a color image by supplying a video signal. Among them, a liquid crystal panel has a large number of liquid crystal elements for controlling transmittance, which are arranged in a plane so as to constitute a screen. In this display principle, RGB three colors may not be disposed at the same position, and hence color filters of RGB three colors are shifted regularly. A minimum unit of each color display is called a sub pixel, and a set of adjacent RGB sub pixels is called a pixel.
In the following description, the terms of pixel and sub pixel may be used in a mixed manner. As described above, for a mechanical reason of the display device, a type or format of color and a position of color should be considered. Practically, misregistration of sub pixels may not be recognized in many cases, but may be perceived as a smoothness of a contour of a letter or a figure in some cases of a mobile terminal or the like with a short visual distance.
A video signal distributed in common is expressed by RGB three colors per pixel or by a signal that may be converted into RGB three colors. For instance, a video signal of a color television is constituted by a luminance signal and two types of color-difference signals, which may be converted into RGB three colors by simple conversion equations.
A video signal distributed in common may have the number of pixels that is different from the number of pixels included in the screen depending on a broadcasting type. For instance, the number of pixels of a color television video signal is different between analog terrestrial broadcasting and satellite high definition broadcasting. Therefore, the number of pixels is converted so that display may be performed on the same display device. In this way, in order to adjust a difference between the input video signal and characteristic of the display device, it is known to perform signal processing such as conversion of a color type, conversion of a color arrangement (sub pixel rendering), and the like.
The liquid crystal panel that performs transmittance control is combined with a light source so as to function as a display. A type of the light source for use is not limited, but the following description exemplifies the case where a light emission unit (backlight) is disposed on the opposite side to the display screen.
A color image to be displayed is generated by combining a wavelength characteristic of the backlight and wavelength characteristics of sub pixels of the RGB three colors. As combined transmittance thereof is higher, light emission of the backlight may be used more efficiently.
JP 60-61724 A discloses a technology for improving the transmittance by providing four color sub pixels including white (W) color in addition to the above-mentioned RGB three colors. Because a color filter is not necessary for W sub pixels, the transmittance is improved three times as compared with the transmittance of RGB sub pixels. The RGB sub pixels and the W sub pixel are arranged in an appropriate order so that the transmittance may be improved.
In addition, JP 2004-538523 A discloses an example of signal processing in which a sub pixel arrangement conversion (sub pixel rendering process) is noted. JP 2004-538523 A discloses a method of performing the signal conversion by filtering process in which the signal conversion method is analyzed from a viewpoint of frequency components in the case where a pixel position of an input signal is different from a sub pixel position of the display panel. The filtering process is a well-known signal processing technology defined by a convolution integral of the input signal and a coefficient, which is characterized in coefficient setting for the purpose of maintaining image quality accompanying a sub pixel arrangement. In addition, when the filtering process is performed, gamma conversion for securing linearity of the signal is essential.
In addition, JP 2004-538523 A discloses, a correction technology for a result of the filtering process operation using a monochrome fine line pattern detection unit on the assumption that sufficient image quality may not be obtained only by the above-mentioned filtering process (blurred fine line or color balance shift).